The use of NMR techniques in measurement, detection and imaging has become desirable in many scientific fields of endeavor. The non-invasive, non-destructive nature of NMR has facilitated application to industrial instrumentation, analysis and control tasks, in a variety of applications, including but not limited to cosmetics, perfumes, industrial chemicals, biological samples and food products. As one example, check weighing is used by the pharmaceuticals industry for monitoring and regulating the amount of drug in a sealed glass vial during filling. The drug weight can be as small as a fraction of a gram, and is required to be weighed with an accuracy of a few percent or better, in a vial weighing tens of grams at a rate of several weighings per second.
International Patent Application No. WO 99/67606 (the “'606 application”), incorporated herein by reference as if fully written out below, describes a check weighing system for samples on a production line using NMR techniques. This system includes a magnet for creating a static magnetic field over an interrogation zone to produce a net magnetisation within a sample located within the interrogation zone, and an RF coil for applying an alternating magnetic field over the interrogation zone to cause excitation of the sample according to the principles of NMR.
As is well known in the NMR art and noted in the '606 application, successful application of magnetic resonance requires that the sample under test be allowed to reach substantially maximum net magnetisation before the application of any further excitation field. Failure to allow the magnetisation to first fully develop will result in a sample reply signal that is less then maximum, and an inaccurate and imprecise resulting weight. Thus, it is imperative to carefully correlate the position of the sample under test with the triggering of further field excitations, such as by the RF coil.
In the '606 application, triggering occurs when the filled vial is at a desired location within the check weighing station. This, in turn, is determined by detecting the moment in time when the filled vial passes an optical position sensor, and the speed of the conveyor belt carrying the filled vial. In other words, the filled vial position is calculated solely as a function of the product of time and velocity. This does not insure that the position of the filled vial at the time of triggering will result in a maximum sample reply signal.
It is desirable to provide a method for triggering further field excitation in a NMR check weighing system for samples on a production line so that the sample under test is positioned to return the maximum sample reply signal.